The present invention relates to a master cylinder.
A master cylinder used in, for example, a brake apparatus of a vehicle may include a bottomed cylindrical cylinder body, a piston slidable in the cylinder body, and a spring assembly urging the piston toward an opening side of the cylinder body. The piston may be formed into a bottomed cylindrical shape having a hole inside the piston, and the spring assembly may be disposed in the hole. A technique for forming a hole of such a bottomed cylindrical piston by the forging method is disclosed in, for example, Japanese Patent Public Disclosure 2002-104164.
If a hole of a piston is formed by the forging method as mentioned above, disadvantageously, an arcuate wall surface portion extending in a curved manner is generated at a joint portion between an axially extending wall portion and a bottom surface of the piston which define the hole. A retainer of a spring assembly inserted into the piston is configured to abut against the bottom surface of the piston. In the piston including the arcuate wall surface portion generated as mentioned above, a diameter of the retainer should be set such that the retainer is spaced apart from the axially extending wall portion of the piston to be prevented from resting on the arcuate wall surface portion.
In this case, however, the seating performance of the retainer may be deteriorated due to interference of the retainer with the arcuate wall surface portion on the most outer end of the bottom portion of the piston. In addition, appropriate centering of the spring assembly by the axially extending wall portion of the piston may not be achieved, whereby the spring assembly may be tilted or displaced as a whole, and the sliding performance of the piston may be adversely affected due to generation of pinching moment at the piston.